Pickling solution



Jan' 23 1945. s. F. URBAN PICKLING SOLUTION Filed April 19, 1941 Patented Jan. 23, 1945 PICKLING SOLUTION *i Stephen F. Urban, Chicago, lll. Application April 19, 1941, Serial No. 389,423

Claims.

The disposal of spent solutions obtained from the pickling of steel with sulphuric acid for the removal of oxide scale therefrom has presented quite a problem.

The "pickling of carbon and low alloy steels is for the purpose of removing oxidic scale and the like from the surfaces thereof,.and includes the immersion of the steel in a tank containing approximately 5 to 7 per cent of sulphuric acid. This results in the solution of the oxidic surfaces thereby producing a surface of clean metal.

As the operation proceeds, the content of ferrous sulphate in the pickling bath increases, with a corresponding decrease in the concentration of the acid. It is, therefore, necessary to make up the acid concentration in the pickling bath by additions of fresh acid. l

Generally speaking, the acid bath may be used eiliciently until itcontains approximately 7 per cent iron or 20 per centl of ferrous sulphate. However, in continuous pickling, per cent of ferrousl sulphate is nearer the maximum desirable, and in most cases it is preferred to discard the bath before the ferrous sulphate reaches this concentration. When the iron content of the bath reaches a permissible limit, the operator begins killing the bath, by adding no more acid, but continuing to use the solution for pickling, increasing the time of the operation,

until the acid is so nearly spent that it acts too, slowly. When the minimum content of free acid 4 is reached, the bath is dumped; that is, discarded for pickling, and anew batch of acid is made up. For continuous pickling operations, substantially new acid may be kept in one of three tanks at all' times, a fourth tank being employed exclusively for killing the acid. Since the spent bath, termed the pickle liquor," cannot be discharged into a stream, various methods have been proposed for treating it. These methods include the steps of neutralizing the free acid with iron and evaporating the water to obtain copperas; treating the liquor to recover the free and combined acid as a more valuable sulphate, such as ammonium sulphate, and the iron as ferric hydroxide; treating the liquor to recover the free acid it co'ntains by crystallizing the ferrous sulphate, which itself presents a disposal problem; or neutralizing the free acid' and precipitating the iron with lime, giving a waste product consisting of a mixture of calcium sulphate and ferrous hydroxide.

This last method limits the cost of other methods, although it promises no returns from the waste acid. To recover the free acid in a pure form at a cost comparable to the market price of new acid is impossible and cheaper methods of recovery necessitate the use of mother liquors for pickling that not only entail changes in pickling practice, but also lower the quality of thel Apickling.

4'It has been noted above that during the pickling operation it becomes necessary to add fresh acid from time to time to the pickling bath in order to maintain a suitable concentration of free acidsin the bath. Since this concentration gives Amaximum eillciency in the operation,`it would be highly desirable to maintain constant. v

The present invention relates to a process `for maintaining continuously a constant concentrathis l concentration tion of sulphuricv acid during pickling, thereby obviating the necessity of adding fresh acid to the pickling bath, and the necessity for the treatment of spent pickle solutions.

In general, the present invention relates to a process for regenerating the sulphuric acid of the pickle bath in a continuous manner by elec-- trolyzing the said bath, using. a mercury cathode Aand preferably, although not absolutely necessarily, an insoluble anode.

By means of the mercury cathode, using any suitable anode and a. proper-.amount of electricv current, the iron can be deposited in the mercury as rapidly as it enters'the solution from the sheet or other body lbeing pickled, there being regenerated thereby an amount of sulphuric acid equivalent to the iron which has been deposited in the cathode, so that there is maintained a' constant acid concentration. y

By the application of this process, which will'4 be described in greater detail hereinafter, itis possible to pickle steel in a much shorter time than has been possible prior to the present improvements, for the following reasons:

The rate at which steel can be pickled at a given teiperature depends upon the concentration of t e acid in the pickleliquor and on the amount of the iron salts contained therein. During the pickling operation, acid is consumed, which reduces its concentration and thereby decreases the rate at which the pickling can be It has been found that with a constant acid concentration, as is maintained in the present process, 2.3 times the amount of steel can be pickled in a given time, as against the usual method of pickling, with a progressively lower acid concentration as pickling proceeds.

Therefore, it |becomes evident that the rate of pickling steel can be increased if means can be provided for removing the iron from the solu tion used for pickling, which iron enters the so lution as a result of the reaction of the acid with steel and/or scale. When the removal of the iron from the solution proceeds as rapidly as the iron enters the solution, with consequent regeneration of the sulphuric acid, there is maintained in the solution an acid concentration of a constant value.

As has been stated previously herein, the removal of iron from pickle liquor and the regeneration of the sulphuric acid is effected by electrolysis of the liquor using a mercury cathode, in conjunction with any suitable anode, the electrolysis being carried on in the. tank in which the pickling is taking place.

However, the removal of iron from the pickle liquor is found to take place very slowly, if at all, unless there is added to the liquor a salt which accelerates the deposition of` the iron into the mercury cathode. It has been [found in practice that salts such as ammonium sulphate and stannous chloride thus accelerate, in some way, the deposition of the iron in the cathode. The ammonium sulphate or stannous chloride may be used either individually or together.

By the addition of the ammonium sulphate and/or stannous chloride it is found that the rate of removal of the iron with attendant regeneration ofthe sulphuric acid is increased very greatly. 'Ihe ammonium sulphate added to the pickle liquor may range, say. from about 3 per cent to about 121/2 per cent, and the amount of stannous chloride may vary from 0.05 per cent to about 0.50 per cent, and the amount of current passed through the pickle liquor may be controlled in accordance with the rate at which it is desired to remove the iron from the solution and regenerate the sulphuric acid.

'I'he amounts of ferrous and/or ferrie iron and the total concentration of iron in the pickle liquor are of no great consequence and may be -maintained at any desired value without interfering with 'the process of removing iron and regenerating acid or without interfering with the plckling process being conducted simultaneously.

'I'he manner in which the present invention may be practiced is illustrated in the accompanying drawing, which shows a diagrammatic perspective view of a combined electrolytic and pickling tank, parts being broken away to show details of construction.

Referring more particularly to the drawing, the combined electrolytic and pickling tank is represented at 3, which contains a tray 5 which is conveniently made of glass or other non-conducting material, which receives the mercury cathode 9. The tray 5 is shown as resting upon mountings 1, which mountings may be composed of glass or any other material inert to the electrolyte, which is indicated at 5. Specimens being pickled are indicated at 8.

A suitable anode II is positioned in the electrolyte above the mercury cathode 9. The anode II preferably is insoluble, and may be composed of lead or carbon, or any other suitable material which preferably is insoluble in the electrolyte. As is illustrated, the anode preferably is a horizontally disposed sheet which rests upon strips I3. The area of the sheet II preferably corresponds to the area of the tray 5.

The anode I I is suspended in the tank by having a portion I1 bent over a side of the tank, a connecting plate, preferably of copper, indicated at I9, being secured to the lbend Il. The connector plate I9 is connected through lead 2| to the positive side of the circuit, current 'being supplied thereto from any suitable source of direct current, which may be a motor generator set, or any other suitable source of current, not shown.

The cathode 9 is connected to the negative side of the circuit through lead 23, and a suitable anrmeter 25 and a voltmeter 21 maybe included in the circuit as shown.

In order to control the temperature of the solution being electrolyzed, a pipe 29 is passed through the tank 3 along its bottom, this pipe 29 being provided with branches 3l and 33 for heating or cooling the electrolyte, one of the branches, for instance branch 3l, being adapted to be connected to a source of cooling water, and branch 33 being adapted to be connected to a suitable source of steam, neither of which sources is shown.

The invention will be understood by reference to the following specific example which illustrates the improved process.

Pickling ,solution amounting to 240 pounds containing 3 per cent of ammonium sulphate was electrolyzed in the apparatus shown on the drawing, using a lead anode and a mercury cathode, the mercury weighing 60 pounds. The bath contained four bars oi' steel 11/4 inches square and 18 inches long each, which bars constituted the specimens to be pickled.

The acid concentration was 3.3 per cent sulphuric acid and the solution contained 3.80 per cent of total iron. Upon electrolysis for four hours and 40 minutes, using a current density of 200 amperes per square foot, the sulphuric acid content showed 2.9 per cent with 3.77 per cent of total iron. Further electrolysis for three hours and 45 minutes at a current density of from to 192 amperes per square foot showed a percentage of sulphuric acid of 2.9 per cent, with 3.86 per cent total iron.

Upon shutting on! the current and allowing the test bars to remain in the solution for seven and one-half hours, the percentage of free sulphuric acid dropped to 1.36 with a total iron content of 4.70. I

It was demonstrated, therefore, that so long as the current passed through the bath, the content of free sulphuric acid and the content of iron in the solution were substantially constant, even though the test specimens were being attacked by the acid. The relationship between the acid 'and iron contents without the passage of current, which is in accordance with the usual practice, is. shown by a reduction of free sulphuric acid in seven and one-half hours of contact with the bars of from 2.9 per cent to 1.36 per cent, and an increase in total iron of from 3.86 per cent to 4.70 per cent.

From the foregoing it will be seen that there is provided a process wherein the iron and sulphuric acid contents of the pickling solution will be maintained substantially constant, the sulphuric acid being generated in the solution directly in the containers in which the pickling is conducted, if desired, thereby not interfering with the pickling operations and providing pickling solutions that are nearly free from iron and of substantially a constant acid concentration, which can be used indenitely, allowing almost continuous pickling. The invention provides a means for maintaining substantially constant acid concentration in the pickling solution.

I claimt 1. The method of pickling ferrous metals, which comprises placing a sulphuric acid pickling solution in a suitable tank, providing a mercury cathode and an insoluble anode in the said solution, adding from about 3 per cent to about 121/2 per cent ammonium sulphate to the solution, and electrolyzing the said solution in the presence of ferrous metal being pickled, thereby depositing iron from the solution in the mercury cathode substantially as rapidly as it enters the solution from the ferrous body being pickled, and regenerating free sulphuric acid in the solution in amounts chemically equivalent to the iron deposited in the cathode, thereby maintaining a substantially constant iron and sulphuric acid content in the solution.

2. The process of pickling ferrous metal, which comprises immersing a ferrous metal body in a pickle solution containing iron sulphate and sulphuric acid, and electrolyzing the said solution in the presence of stannous chloride, and in the presence of a mercury cathode and an insoluble anode, depositing iron on the mercury cathode substantially as rapidly as it enters the solution from the ferrous body being pickled, thereby maintaining the contents of iron and sulphuric acid in the solution substantially constant during the pickling operation, the stannous chloride being employed in amounts ranging from approximately 0.05 per cent to approximately 0.5 per cent.

3. The method of pickling ferrous metals, which comprises placing pickling solution containing iron sulphate and sulphuric acid in -a tank in the presence of ferrous metal to be pickled, providing a mercury cathode and an insoluble anode in the said solution, electrolyzing the solution in the presence of salts which accelerate the deposition of iron in the mercury cathode with regeneration of sulphuric acid in the solution in amounts chemically equivalent' to the iron deposited in the cathode, the said saltsl being ammonium sulphate and stannous chloride, the ammonium sulphate being added in amounts ranging from approximately 3 per cent to approximately 12.5 per cent, and the stannous chloride being employed in amounts ranging from approximately 0.05 per cent to approximately 0.5 per cent.

4. The process of pickling ferrous metal, which comprises immersing a ferrous metal body in a pickle solution containing iron sulphate and sulphuric acid, and electrolyzing the said solution in the presence of salts which accelerate deposition of iron, and in the presence of a mercury cathode and an insolubl-e anode, depositing iron in the mercury cathode substantially as rapidly as it enters the solution from the ferrous metal body being pickled, thereby maintaining the contents of iron and sulphuric acid in the solution substantially constant during the pickling operation, the said salts which accelerate deposition of iron being ammonium sulphate and stannous chloride, the ammonium sulphate being employed in amounts ranging from approximately 3 per cent to approximately 12.5 per cent, and the stannous chloride being employed in amounts ranging from approximately 0.05 per cent to approximately 0.5 per cent.

5. The process of pickling ferrous metal, which comprises immersing a ferrous metal body in a pickle solution containing iron sulphate and sulphuric acid, electrolyzing the said solution in the presence of salts accelerating deposition of iron and in the presence of a mercury cathode and an insoluble anode, depositing iron from the solution in the mercury cathode substantially as rapidly as 'it enters the solution from the ferrous metal body being pickled, thereby maintaining the contentsof iron and sulphuric acid in the solution substantially constant during the pickling operation, and continuously maintaining in the bath the metal to be pickled, the said salts employed for accelerating deposition of iron being ammonium sulphate and stannous chloride, the ammonium sulphate being employed in amounts ranging from approximately 3 per cent to approximately 12.5 per cent, and the stannous chloride being employed in amounts ranging from approximately 0.05 per cent to approxi- STEPHEN F. URBAN. 

